PPAR-gamma Signaling in Normal Pilosebaceous Units (PSU) and in Scarring Alopecia ABSTRACT Primary cicatricial or scarring alopecia (CA) are characterized by a folliculocentric inflammation with the ultimate replacement of the follicle with fibrous tissue and progressive and permanent hair loss. However, the cause of the inflammatory attack and the molecular pathogenesis has yet to be elucidated. Our studies with the lymphocytic CA, Lichen planopilaris (LPP), have yielded novel results which provide clues to disease pathogenesis. We have recently shown that PPAR signaling is lost in lichen planopilaris (LPP), a lymphocytic CA, and that targeted deletion of PPAR in stem cells of the hair follicle causes scarring alopecia. However, the mechanisms responsible for loss of PPAR signaling in LPP are not understood. Our new data shows that the Aryl Hydrocarbon Receptor (AhR), best known for mediating the toxicity of dioxin, is significantly upregulated in LPP and in the PPAR KO mouse. Furthermore, a mutually antagonistic regulation exists between PPAR and AhR in hair follicle outer root sheath (ORS) cells in vitro. We show that the expression of stem cell markers (LGR5, LHX2, SOX9, TCF3) is decreased and that cytokines, growth factors and tissue remodeling genes are upregulated in LPP and in the PPAR KO mouse. We hypothesize that functional interplay of PPAR with AhR has a central role in pilosebaceous unit (PSU) homeostasis, hair follicle stem cell kinetics and disease progression in CA. The overall goals of this proposal are to elucidate the mechanisms by which AhR modulates PPAR and to delineate the mechanistic effects of this cross-talk on hair follicle outer root sheath (ORS) cells in vitro and during disease progression in scarring alopecia. We will test this hypothesis using a combination of human tissue (normal, unaffected and affected primary CA biopsies), ORS cells, the PPAR stem cell specific KO mouse model that we have developed and the K14-AhR transgenic mouse. In Aim 1, we will test the hypothesis that a mutual functional repression exists between AhR and PPAR and that aberrant PPAR signaling is involved in the pathogenesis of all CA. The focus of Aim 2 is to investigate the mechanistic effects of PPAR- AhR cross-talk on hair follicle ORS cells in vitro and its implications for scarring and fibrosis in CA. In Aim 3, we will determine the effects of stem cell specific PPAR ablation or AhR constitutive activation on the PSU and in the development of scarring alopecia. The effect of these changes on hair follicle stem cell kinetics will be investigated. The efficacy of PPAR agonists to modify immune activation toward normalcy and restore normal hair function and re-growth in the animal model will be tested. These studies provide a novel framework for understanding the role of PPAR in the pathophysiology of primary CA, they provide novel diagnostic biomarkers, facilitate the classification of clinically distinct lymphocytic and neutrophilic CA and suggest potential new therapeutic strategies, thereby addressing an important medical need. PHS 398/2590 (Rev. 11/07) Page Continuation Format Page